Zeolites are powdery crystalline substances that are widely used as catalysts in industrial processes.
To achieve a certain level of crushing strength for industrial use, zeolite power is integrated through a binding substance called a binder and is shaped into a size that is easy to handle. However, such shaped catalysts generally have lower performance than they show in the original powder form. This reduction in performance is probably due to alterations of or interferences to the catalytically active sites in the zeolites by the interaction or chemical bonds with the binders. Further, starting material and products are diffused more slowly in the shaped catalysts than in vacuum or organic solvents. As a result, the supply rate of starting material to the catalytically active sites cannot keep up with the reaction rate. In addition, the product is caused to stay longer in the catalyst and consequently side reactions are accelerated, a state called diffusion-limited state. In particular, the limited diffusion results in accelerated accumulation of cokes in the case of highly active catalysts or under high temperature or high pressure where the reaction rate is high. As a result, the shaped catalysts drastically reduce their activity and life, which are the most important properties in the use of catalysts. Further, the binders sometimes work as catalysts to cause side reactions.
Countermeasures to minimize the reduction in performance of the zeolite shaped catalysts include reducing the usage amounts of binders and increasing the pore volume of the shaped catalysts. The binders are selected from compounds that are inactive in reactions to be catalyzed by the shaped catalysts and do not deteriorate the reactivity of the zeolites. Decreasing the usage amount of binders reduces the crushing strength of the shaped catalysts and makes the shaping difficult. Increasing the pore volume reduces the crushing strength of the shaped catalysts. Further, shaping properties are another problem that should be solved to obtain shaped catalysts having desired properties. For example, extrusion methods entail that a mixture of zeolite, a binder material, water and a shaping auxiliary has an appropriate viscosity for the mixture to be shapeable with an extruder. The mixture cannot be extruded with an excessively high viscosity and cannot be shaped with an excessively low viscosity.
The shaping auxiliaries are for example thickening agents, surfactants, water retention agents, plasticizers and organic binders. Selection of the shaping auxiliaries is difficult because not only shaping properties but also freeness of adverse effects on performance of the shaped catalysts should be considered.
A large number of methods for solving the above problems have been disclosed.
For example, Patent Documents 1 to 4 disclose methods for forming adsorbents or catalysts. These patent documents involve carboxymethyl celluloses as shaping auxiliaries, and binders used therein are inorganic compounds which are bound with zeolites relatively easily, such as acid clay, Kibushi clay, sepiolite, attapulgite and kaolin. These inorganic binders contain large amounts of impurities other than alumina components and the like and can deteriorate the catalytic performance of zeolite in some reactions. Accordingly, they should be removed from the shaped catalysts in a separate step and the production costs are increased.
Patent Document 5 discloses a process of producing a shaped catalyst from a siliceous solid and zeolite in the presence of an alkali metal compound. In the working examples disclosed therein, high crushing strength (12.7 kg/cm) and high pore volume (0.615 cc/g) are obtained with the pore diameters concentrated in 200 to 600 Å (20 to 60 nm) when the content of the silica binder in ZSM-5 zeolite shaped catalyst is relatively low at 35.5%. According to this process, siliceous solids are used as binders and thereby the zeolite shaped catalysts are free from substances that will deteriorate the catalytic performance. However, the crystallinity of the aluminosilicate zeolite can be reduced in the presence of alkali metal compounds, possibly resulting in lower catalytic performance. Further, treatments with ammonium nitrate and nitric acid are required in a later stage of the catalyst production to remove the alkali metals. Thus, this process is not economical.
Patent Documents 6 to 8 disclose extrusion methods for shaping zeolites wherein the use of alkali metals is eliminated by using amine compounds as shaping auxiliaries. Silica sol is used as a silica binder material that does not deteriorate the performance of the zeolite catalysts. This method has been shown to maintain substantially intact the crystallinity of zeolite in the shaped catalysts and to provide high crushing strength of the shaped catalysts. However, according to the working examples disclosed therein, the content of silica binder in the shaped catalysts is high ranging from 40 to 50 wt %, and great reduction in shaped catalyst performance is inevitable.
Patent Documents 9 and 10 disclose processes for producing titanium oxide-supported catalysts capable of decomposing and removing harmful components in incinerator exhaust gases, in which processes easily thermally decomposable substances such as acetal resins, acrylic resins, polyester resins and methacrylic acids are used as porosifiers whereby pore sizes in the range of 300 to 450 nm are created. In these processes, the porosifiers are decomposed during the production. According to the working examples disclosed therein, resins having a particle diameter of 3 μm should be used in order to obtain pore sizes in the above range. That is, the porosifier particle diameters are greatly different from the target pore sizes. Therefore, it is expected that controlling the pore volumes or pore diameters will be difficult and creating pores in uniform sizes will be even more difficult.
As described above, the use of carboxymethyl celluloses as shaping auxiliaries in combination with binders such as kaolinite and acid clay which are easily bound with zeolites can result in deteriorated performance of the shaped catalysts. The silica binders do not greatly reduce the shaped catalyst performance but do not have good shaping properties with zeolites. By the use of alkali metals in extrusion material mixtures, the mixtures can be extruded to give shaped catalysts having a sufficiently high pore volume and high crushing strength. However, it also entails a step for removing large amounts of alkali metals and adds costs; further, controlling properties of the shaped catalysts is difficult. The use of amine compounds as shaping auxiliaries allows for extrusion shaping under a weakly alkaline atmosphere. However, the binder content has to be increased to a certain level in order to obtain sufficient crushing strength, and consequently the reduction in performance such as catalytic activity and life of the shaped catalysts cannot be avoided.                Patent Document 1: JP-A-S61-155216        Patent Document 2: JP-A-S61-242911        Patent Document 3: JP-A-H02-157118        Patent Document 4: JP-A-H10-087322        Patent Document 5: JP-A-S61-026509        Patent Document 6: JP-A-H04-346839        Patent Document 7: JP-A-H06-211517        Patent Document 8: JP-A-2003-510181        Patent Document 9: JP-A-2002-079087        Patent Document 10: JP-A-2002-136871        